I wish someone would have recommended him to tweak one of many available OSM styles in TileMill. He also could have exported the layer in any tile shape, projection, and resolution using mapnik. You can even render to vector trivially, which comes in very handy for any printable pdfs.
I've been using Mapnik for about 8 years now, I think, since it was the first alternative to MapServer. OSM has always been using Mapnik, with mapnik styles in CartoCSS for a long time too (http://wiki.openstreetmap.org/wiki/CartoCSS). TileMill beta (https://github.com/mapbox/tilemill/tree/v0.5.1) came out 4 years ago. So, I definitely would have made the same recommendation 1.5 years ago. It still came out pretty cool :)
Feel free to ask me any gis or web mapping questions as demiurge@irc.freenode.org, I'd love to facilitate more beautiful maps out there.